The present invention relates to a two-speed accessory drive for use in driving a plurality of engine accessories at either one of two selected predetermined speed ratios relative to the spped of an input shaft. More particularly the invention relates to an accessory drive which utilizes internally expanding, centrifugally responsive, lever actuated brake shoes.
The present invention finds application for activating a plurality of engine driven accessories by utilizing a drive or clutching device which utilizes a plurality of internally expanding, self energizing brake shoes. An advantage of utilizing the internally expanding brake shoes in conjunction with its operating mechanism as described below is to remove any thrust loading on the various bearings of the drive. The self energizing feature of the present invention permits the drive to be activated by using a minimum actuating force. Prior two speed accessory drives are configured to drive the associated accessories at a first speed relatively equal to the speed of the input shaft or engine and a at second or higher speed as a function of the operating characteristics of the engine. These prior devices are characterized by an excessively high degree of gear whine which is generated by a planetary gear set which is often used within such devices. In contrast the present invention provides an efficient and quietly operating two speed drive. Another object of the present invention is to control the effective gear ratio of a planetary gear set through the application of a plurality of internally expanding, self-energizing brake shoes. Accordingly it is an object of the present invention to provide a drive or clutching device that is capable of driving accessories at either of two preselected speeds. Accordingly, the invention comprises: a two speed accessory drive operative to drive an output member at either of two predetermined speeds relative to the speed of input member or shaft. The drive comprises a drum rotatable with the input shaft and includes an engagement surface. A planetary gear set is positioned remote from the drum and comprises a sun gear, a plurality of planet gears drivingly linked to the sun gear and supported by a planet carrier and a ring gear driving positioned about the planet gears. The sun gear is drivingly connected to the input shaFt and the planet carrier is operatively connected to the output member. The drive further includes means for rotationally isolating the planet carrier and the output member from the input shaft and engagement means comprising: a shoe disk positioned within the drum and drivingly connected to the ring gear. The engagement means further includes a plurality of centrifugal acting, brake shoes rotatable with and pivotably supported relative to the shoe disk and means for moving the brake shoes to a first or engaged condition to engage the brake shoes to the drum and for moving the brake shoes to a second or disengaged condition to disengage the brake shoes from the drum, and a plurality of levers one for each brake shoe, pivotably connected to and for moving a corresponding one of the brake shoes. The engagement means also includes means for moving the levers relative to the shoe disk to move the brake shoes to the first and second conditions. The drive further includes means for selectively stopping the rotation of the ring gear during instances where the brake shoes are disengaged from the drum for permitting the planet carrier to rotate at a speed less than the speed of the input shaft.